For complex precision optical assemblies, such as the prism assemblies in displacement measuring interferometers or cube corner assemblies, it is frequently required to know and control to extremely tight tolerances the relative positions of reflective surfaces, dihedral angles, and the location of intersections and vertices. Typically, this is achieved using complex combinations of interferometers and/or autocollimators, which define orientations of surfaces, and mechanical indicators which are used, with appropriate mechanisms, to measure displacements. Sometimes mechanical stops are used to provide orientation or position references. See, for example, Peter G. Halverson, et al., Progress Towards Picometer Accuracy Laser Metrology For The Space Interferometry Mission, International Conference of Space Optics, ICSO Dec. 5, 6, 7, 2000, Toulouse, France and E. Schmidtlin, “Wide-Angle, Open-Faced Retroreflectors for Optical Metrology”, Photonics Tech Briefs, 23, 3, pp. 15a–16a, 3, 1999. The tolerances on currently required space flight qualified opto-mechanical assemblies are considerably more demanding than those described in the foregoing cited references.
Prior approaches to the metrology of precision optical assemblies such as hollow corner cubes have a number of disadvantages. Principally, they rely on a sequence of independent measurements of multiple degrees of freedom, often using very different metrology tools, all of which must be related to achieve the desired end result, a process that carries with it considerable uncertainty. For example, separate measurements of angles and displacement invite errors due to drift. Further, if the surfaces of the objects to be assembled into the complex optical system are coated with a soft material (e.g., gold), then mechanical indicators and/or locations can cause damage to these coatings. The most common optical devices for measurements during assembly, such as conventional laser Fizeau interferometers, lose track of absolute position if the measurement beam is broken while manipulating the orientation of the assembly under test. To overcome some of these difficulties, one often has recourse to an optical fixture such as a master prism to facilitate multiple surface measurements simultaneously, but these optical fixtures may be themselves unreliable for very high precision assemblies.
Accordingly, it is a primary object of this invention to provide convenient apparatus and methods by which complex ultra high performance space optic assemblies may be measured and/or assembled.
Other objects of the invention will be obvious and will appear hereinafter when the detailed description is read in connection with the accompanying drawings.